Electrochemical cells containing an anode, a cathode and a solid, solvent-containing electrolyte incorporating a salt are known in the art and are usually referred to as "solid batteries". These cells offer a number of advantages over electrochemical cells containing a liquid electrolyte (i.e., "liquid batteries").
Typically, solid batteries employ a solid electrolyte interposed between a cathode and an anode. The solid electrolyte contains either an inorganic or an organic matrix and a suitable salt, such as an inorganic ion salt, and preferably an electrolyte solvent as separate components. The inorganic matrix may be non-polymeric, e.g., .beta.-alumina, silver oxide, lithium iodide, and the like, or polymeric, e.g., inorganic (polyphosphazene) polymers, whereas the organic matrix is typically polymeric. Organic polymeric matrices for use as solid electrolytes are well known in the art and are typically organic polymers obtained by polymerization of a suitable organic monomer as described, for example, in U.S. Pat. No. 4,908,283. Suitable organic monomers include, by way of example, ethylene oxide, propylene oxide, ethyleneimine, epichlorohydrin, ethylene succinate, and an acryloyl-derivatized alkylene oxide containing at least one acryloyl group of the formula CH.sub.2 =CR'C(O)O-- where R' is hydrogen or a lower alkyl of from 1-6 carbon atoms.
The preferred solid polymeric electrolyte is typically composed of an inorganic ion salt, a low molecular weight compatible solvent and a solid polymeric matrix which is formed by polymerization of an organic monomer or prepolymer. A distinction is made in the art between those solid electrolytes which contain a low molecular weight solvent (i.e., a solvent electrolyte or plasticizer) and those solid electrolytes which do not contain such solvents, Fiona M. Gray, "Solid Polymer Electrolytes", ibid., pages 1-2 and pages 108-109.
Solid polymeric electrolytes have many advantageous properties for the fabrication of electrochemical cells and batteries such as: ionic conductivity, thermal stability, reduced corrosion of the electrodes, cyclability, mechanical flexibility, compactness and low self-discharge rates. Solid polymeric electrolytes permit us to create electrochemical sources of high energy per unit weight. Solid electrolytes and particularly polymeric electrolytes have a principal advantage in being prepared in thin layers which reduces cell resistance and allows large drains at low current densities. The subject has been treated in several recent publications, i.e. Fiona M. Gray, "Solid Polymer Electrolytes", VCH Publishers, Inc., New York, 1991; and M. Gauthier, et al., "Solid Polymer Electrolyte Lithium Batteries", Chapter 9, in "Polymer Electrolyte Reviews", eds. J.R. MacCallum and C.A. Vincent, Elsevier, N.Y., 1989, which are incorporated herein by reference in their entirety.
Thus, it is stated in the art that the polymeric electrolyte plays several roles in the solid polymer battery. First, it is an ionic conductor that can be made very thin to improve the energy density of the battery. It is also a flexible mechanical interelectrode separator which eliminates the need for an inert porous separator. Finally, it is a binder and an adhesive which ensures good mechanical and electrical contact between the electrodes. The present invention adds several additional benefits to the art of solid polymeric electrolytes.
The solid polymeric matrix of which the solid polymeric electrolyte is composed is preferably an organic polymer. The prior art favors poly(ethylene oxide) of molecular weight from 1,000 to over 100,000, substituted with cross-linkable groups such as acrylates and vinyls. Cross-linking is achieved by thermal or radiation treatments of the polymer, U.S. Pat. Nos. 4,908,283, 4,830,939 and 5,037,712. Chemical cross-linking has also been suggested, U.S. Pat. No. 3,734,876.
Besides poly(oxyethylene) homopolymers, i.e. poly(ethylene oxide), it has been found that copolymers containing poly(oxyethylene) groups can be used as in solid polymer electrolytes when copolymerized with siloxanes, K. Nagaoka, et al., J.Polym. Sci. Polym. Lett. Ed.,22, 659 (1984); Phosphazene, P.M. Blonsky, et al., J. Am. Chem. Soc., 106, 6854 (1984); Urethanes, A. Bouridah et al., Solid State Ionics, 15, 233 (1985); or Cross-linked with Phosphorous Oxychloride, J.R.M. Giles, et al., Solid State Ionics, 24, 155 (1987), Polym. Commun. 27, 360 (1987). Phosphazene monomers and the resulting polyphosphazene solid matrix are disclosed by Abraham et al., Proc. Int. Power Sources Syrup., 34th, pp. 81-83 (1990) and by Abraham et al., J. Electrochemical Society, Vol. 138, No. 4, pp. 921-927 (1991). Phosphonitrilic polymers, or phosphazene polymers, as they are also called, are inorganic-type polymers of recent discovery. The repeating backbone unit of the polyphosphazines, (N=P), displays their inorganic character, M. P. Stevens, "Polymer Chemistry," 2nd Edition, Oxford Press, N.Y., 1990, pages 494-496; Gray, "Solid Polymer Electrolytes", ibid., pages 97-98. At page 103, Gray discloses an amorphous network system based on phosphate ester crosslinks of polyethylene glycol brought about by the reaction of POCl.sub.3 with poly(ethylene oxide) glycol; J.R.M. Giles et al., Polym. Commun. 27 (1987), p. 360, and Solid State Ionics 24 (1987), p. 155.
In the design of solid polymeric electrolytes both the properties of ionic conductivity and mechanical strength must be provided. It has been found advantageous to incorporate inorganic ion salt and low molecular weight organic solvents into the solid electrolytes, as well to select polymers which enhance ionic conductivity. Cross-linking of the polymers can lead to stronger solid electrolytes, i.e. resilient thin layers of electrolyte, but cross-linking must not be to the detriment of ionic conductivity. Thermal and radiation-induced cross-linking (curing) have been extensively used for this purpose. U.S. Pat. No. 4,654,279 describes a two-phase solid polymeric electrolyte consisting of an interpenetrating network of a mechanically supporting phase consisting of cross-linked polymers, and a separate ionic conducting phase consisting of a metal salt and a complexing liquid therefor which is a poly(alkylene oxide). Poly(alkylene oxide), derivatized with acryloyl and urethane groups, is a polymer precursor for radiation-cured solid polymeric electrolytes. However, radiation-cured solid polymeric electrolytes may lack sufficient mechanical strength and toughness.
It would be advantageous if a solid polymeric matrix had the above-identified properties without the necessity of a separate curing step.
It would be advantageous if the solid polymeric matrix was a flame-retardant material. By placing a flame-retardant electrolyte in direct contact with the highly reactive lithium anode, an extra measure of safety is achieved.